Preparation of electret transducer elements by application of controlled breakdown electric field

ABSTRACT

A thin film electret with excellent surface charge properties is produced by placing a thin polymer film between two electrodes together with a dielectric plate and by applying a voltage of about 30 keV across the resulting sandwich of elements for about 1 minute. The process is carried out at room temperature and at atmospheric pressure. By using an auxiliary dielectric plate that is much thicker than the film and has the proper resistivity, a high voltage drop can be applied across the film without a resulting destructive breakdown. Charge-densities of up to 1.5 X 10 6 C/cm2, which are greater by a factor of 3 than those previously reported, are produced.

United States Patent SUPPLY Sessler et all l {451 Dec. 5, 1 972 [541' PREPARATION OF ELECTRET' I [56] References Cited TRANSDUCER ELEMENTS BY v UNITED STATESPATENTS APPLICATION OF CONTROLLED 3 354 373 "967 F t 8 T E a OVIC BREAKDOWN EL CTRIC FIELD 3,449,094 6/1969 Baxtetal ..307/88 ET [72] Inventors: Gerhard Martin Sessler, Summit;

James Edward west, plainfield, Primary Examiner-Stanley M. Urynowicz, Jr. both of N1 Attorney-R. J. Guenther and William L. Keefauver [73] Assignee: Bell Telephone Laboratories, lncor- TR! porated, Murray Hill, NJ. [57] ABS CT A thin film electret with excellent surface charge pro-' [22] N 1970 perties is produced by placing a thin polymer film [21] Appl. No.: 85,882 between two electrodes together with a dielectric I o r v plate and by applying a voltage of about 30 keV across the-resulting sandwich of elements for about 1 [52] US. Cl. .-......307/88 ET, 179/111 E, 29/594, minute The process is carried out at room tempera 195 ture and at atmospheric pressure. By using an auxilia- [51] Int. Cl. ...H04r 19/04 ry dielectric plate that is much thicker than the film [58] 1 Field of Search ....307/88 ET; 320/ 1; 340/ 173.2; and has the proper resistivity, a high voltage drop can 29/ 19S,25.4l, 25.42, 594'; 179/ I'll E; be applied across the film without a resulting destruc- 161/216 tiye breakdown. Charge-densities of up to 1.5 X 10' i f v C/c'm which are greater by a factor of 3 than those previouslyreported, are produced. 11 Claims, 3 Drawing Figures METAL //y///// ELECT RODE 2 A I 20 4 l0 0F. L

-4' POWER g L E T wE PREPARATION OF ELECTRET TRANSDUCER ELEMENTS BY APPLICATION OF CONTROLLED BREAKDOWN ELECTRIC FIELD I fabrication of. film electrets that have superior charge retention characteristics, both indry and in humid atmospheres, that are essentially immune to temperature variations, and that exhibit extremely uniform charge densities.

I I BACKGROUND OF THE'INVENTION I Dielectric materials in the form ofthin films, forexample, of various polymers such as polyesters, one'of which is known. by the brand name Mylar, various fluorocarbons, such as the one available commercially with the brand name Teflon, various polycarbonate resins and the like, are used extensively in a variety of applications. For example, they are used in electroacoustic transducers-in the fabrication of capacitors, precipitators, dielectric storage elements, and the like. In a typical electroacoustic transducer, such as an electrostatic microphone or loud speaker, for example, a thin film of such a material is employed as the vibrating element. To avoid the need for external bias, the moving film is permanently polarized or charged; when so charged, it is known as an electret film. Film electret transducers, for example, of the sort described in Sessler-West US. Pat. No. 3,118,022, granted on Jan.

' 14, 1964, have all of the advantages of conventional capacitor units but .virtually none of their disadvantages. Unlike the capacitor transducer, an electret unit requires no separate power supply and is mechanically much simpler. ltalsohas a higher capacitance which allows greater freedom in circuit design. Perhaps even more importantly, it has high sensitivity, good frequency response, and low distortion.

Several methods of producing permanent electric charges on dielectric materialshave been described in the art. Among these are thermal procedures using the simultaneous application ofheat and an electric field, corona and Townsend discharge methods, and electron bombardment techniques using electron beams. Due to charge realignment, charge separation, or charge injection, these methods yield electrets characterized by a heterocharge, a homocharge, or a combination of both.

' One of the most widely used arrangements for charging polymer films employs a system for impressing a relatively high voltage across a film held between a pair of electrodes. Typically a dielectric insert is-sand- SUMMARY OF THE INVENTION It is thus an object of this invention to fabricate thin film electrets which exhibit extremely high charge densities which exceed those achieved using prior art charging techniques. It is another object to assure large full trap densities in thin film electrets.

These and other objects are achieved in accordance with the invention by applying 'a' field, of sufiicient strength to cause breakdown in the polymer films, to a sandwich arrangement consisting of the polymer film which has athin conductive layer bonded to one of its surfaces (the resulting film and conductive layer is commonly called a metalized foil), and another much thicker dielectric material of selected conductivity. By proportioningthe dimensions of the dielectric insert and by selecting the material with the proper conductivity, the insertwithstands the excitation applied to a' pair of encompassing electrodes and does not itself breakdown. Itis nonetheless able to controlthe actual breakdownof the polymer foil short of its destruction. Noair gaps are needed; the elements of the sandwich may be held tightly together. By eliminating all air gaps, the possibility of nonuniform charging is also reduced. Processing'in accordance with the invention may take place at room temperature and yet produce a uniform homocharge in the foil of high density and long life.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be fully apprehended from the following detailed description of illustrative embodiments thereof taken in connection with the appended drawings in which:

FIG. 1 illustrates in simplified form the construction of an electrostatic acoustic transducer which illustrates the manner in which a thin film electret is utilized as the vibrating element;

FIG. 2 illustrates schematically a suitable" arrangement for preparing thin film electrets in accordance with the invention, and

FIG. 3 illustrates effective surface charge as a function of time after charging on various thin film electrets prepared in accordance withtheprinciples of the invention.

DETAILED DESCRIPTION OF THE INVENTION transducer which employs a thin film electret is shown wiched together with the film between the electrodes in FIG. 1. It consists of an'electrically charged plastic film 10 stretched over a metal backplate 11. Typically, film 10 is composed of a polymer material, such as, polyethylene terephthalate (PET), known commercially as Mylar, or polyfluoroethylenepropylene, known commercially as Teflon FEP or Teflon TFE, and polycarbonate (PC) with a thickness of about 0.001 inch. An even thinner metal layer 12 is evaporated onto the side of the film not facing the backplate, i.e., the outer surface of film 10. The resultant film and conductive layer is termed a metalized foil. The backplate surface is generally arranged so that the foil contacts its surface at discrete points or at long discrete lines only.

In the areas where no contact is made, shallow pockets permit it to vibrate when sound waves impinge on it. In addition, backplate 11 is perforated and supported above an air cavity 14. This arrangement reduces the stiffness of the air cushion behind the diaphragm and lets the film vibrate with a greater amplitude, thus to increase transducer sensitivity.

Because of the permanent charge on foil 10 12, an electric field is established between the foil and backplate '12. The foil and backplate are connected, by means not shown, to a high impedance input circuit. Motion of the foil, as a result of an impinging sound wave, for example, causes a small voltage to be generated in the input circuit. This voltage is proportional to sound pressure.

, Reproducibility of the sensitivity of the electrostatic transducers discussed above, together with high sensitivity, good frequency response, and low distortion, depends, in large measure, on the uniformity of charge distribution throughout the body of the electret diaphragm, on the density of the electret charge, and on the ability of the electret to retain charge despite external influences. Thus, a useful lifetime depends on the low decay rate of the electret charge.

Electret films with the requisite properties for use as the vibrating element of an electrostatic transducer, as well as for other electret applications, are fabricated in accordance with the invention by applying a controlled electric field to a polymer foil sufficient to establish a voltage drop across the foil in excess of its breakdown voltage. FIG. 2 schematically illustrates a typical charging configuration.

As illustrated in FIG. 2, two metal electrodes, 21 and 22, each typicallyabout 6 centimeters (cm) in diameter are employed to encompass a polymer foil 10 l2 and an auxiliary dielectric insert 23. Typically, the foil is about 2 25 micro-meters (pm) thick and the dielectric plate 23 is formed of one or two sheets of soda-lime glass, each approximately 0.1 cm thick. Preferably the elements of the sandwich between the electrodes are held in close proximity to one another so that any irregular air gaps between the elements are the result of surface roughness only. For clarity of exposition, the extent of the air gaps between the elements shown in FIG. 2 is greatly exaggerated.

A voltage of about 30 kilovolts (kV) from a conventional power supply 20 is applied across the sandwich by way of electrodes 21 and 22 for a period of about 1 minute. This interval of application has been found to be optimum in practice. By properly choosing the voltage and the dielectric inserts, the current density will be in the range of l to Amperes per square centimeter (A/cm), which was found optimal for this process. This value is-much above a typical breakdown current for most polymer materials. A large part of the current flows through localized channels in the polymer and charge deposition probably occurs in areas remote from these channels. That is to say, breakdown occurs at discrete locations only. Yet, the remainder of the foil is subjected to a prebreakdown condition and charge deposition occurs primarily in the remainder; i.e., in areas of the foil remote from the point of breakdown. Application of heat is not necessary; application at normal room temperature is perfectly satisfactory. Following application, the sign of the charge on polymer film 10 corresponds to a homocharge. Depending upon the polarity of the applied electric field, the film has a net positive or net negative charge on its nonmetallized side. Of course, a voltage in the prebreakdown range across the film also produces some charge deposition but the resulting electret charge density is correspondingly smaller.

A glass insert 23 is preferred for controlling polymer breakdown because it has a resistance sufficiently high, on the order of 10 to 10 ohms for an area of 10 to cm to prevent the breakdown from becoming destructive, yet low enough to allow a sufficiently high voltage to develop across the polymer. Furthermore, a glass insert makes the spacing of the electrodes relatively uniform and thus allows a greater field to be applied across the polymer for the same electrode smoothness than would be possible without an insert. It has been found in practice that the field in a dielectric insert 23 composed of layers of soda-lime glass is on the order of 250 kV/cm, which is a value in the prebreakdown range.

The greatest effective surface charge densities found for three representative polymer films of various thicknesses, each metallized on one side and charged in accordance with the invention, are tabulated below. Densities, as measured by a nondestructive method, for both charge polarities are shown. These values, obtained about 60 seconds after the termination of the charging process, are given as deviations from a nominal value of 10' C/cm In addition, full trap densities for these materials, calculated from the charge densities, are shown in the last column of the table.

The internal fields corresponding to the greatest charge densities, as shown in the table, are, for example, 4 X 10 V/cm for PET and are thus just below the breakdown fields. The greatest effective surface charge does not increase with foil thickness because the charges are initially trapped within a narrow layer, probably close to the surface.

Surface charge as a function of time of several PET and FEP electrets stored in a desiccator is illustrated by way of example in F IG. 3. Four to 8 months after charging the negative and positive charges on PET are still about 2 to 3 times greater than on foils produced with corona discharge or by electron bombardment. Positively charged FEP electrets have been found to be subject to a more rapid charge decay than the negatively charged ones.

As opposed to thermal electrets, low temperature currents due to depolarization of heterocharges are very minute. The absence of a large heterocharge is attributed to the fact that charging takes place at room temperature. Moreover, the high temperature current due to homocharge depolarization from PET charged foil and one of said electrodes, said dielectric insert having a resistance sufficiently high to permit nondestructive breakdown of said foil, yet low enough to allow a sufficiently high voltage to be developed across said foil, said foil, auxiliary dielectric insert, and encompassing electrodes being held together, means for applying a voltage between said electrodes high enough to cause nondestructive breakdown in said foil in localized channels and low enough to cause prebreakdown in said insert, and means for maintaining said voltage for an interval of about one minute to charge areas of said foil remote from said channels. 2. Apparatus for electrically charging a thin polymer foil, as defined in claim 1, wherein,

said auxiliary dielectric insert comprises at least one sheet of soda-lime glass approximately 0.1 cm thick. 3. Apparatus for electrostatically charging a thin polymer foil, as defined in claim 1, wherein,

the thickness of said polymer foil and the thickness of said auxiliary dielectric insert are proportioned to establish a steady state current density within the critical range of about 10'? to 10 A/cm. 4. Apparatus for electrostatically charging a thin polymer foil, as defined in claim 1, wherein,

said auxiliary dielectric insert has a resistance in the range 10 to 10 ohms for an area of 10 to 100 cm 5. Apparatus for electrostatically charging a thin polymer foil, as defined in claim 1, wherein,

said auxiliary dielectric insert has a resistance sufficiently high to induce breakdown of said foil but low enough to allow a relatively high voltage to be developed across said polymer foil.

6. Apparatus for electrostatically charging a thin polymer foil, as defined in claim 1, wherein,

said voltage applied between said electrodes is of approximately 30 kV.

7. The method of fabricating a foil electret characterized by a charge density in excess of 0.5 X 10' C/cm, which comprises the steps of:

sandwiching a foil of polymer material and'an auxiliary dielectric plate having a resistance sufficiently high to permit nondestructive breakdown of said foil, together between a pair of plane concoi gge i g ai il ctrodes to hold said sandwich of foil and dielectric in close contact, applying a sufficiently high voltage between said electrodes to cause said foil to break down in localiz ed channels, but not sufficiently high to cause said dielectric plate to break down, and maintaining said voltage between said electrodes for aninterval of about one minute to charge areas of said foil remote from said channels.

8. A foil electret characterized by a charge density in excess of 0.5 X 10' C/cm prepared in accordance with the method defined in claim 7.

9. The method of fabricating an electret transducer, which comprises the steps of:

preparing an electret diaphragm from a thin polymer film metallized on one of its surfaces by supporting said film between two electrodes,

inserting a dielectric plate having a resistance sufficiently high to permit nondestructive breakdown of said film between the nonmetallized surface of said film and one of said electrodes, and supplying a voltage of approximately 30 keV between said electrodes for about 1 minute; and

supporting said electret diaphragm in juxtaposition to a conductive backplate. 10. The method of fabricating an electret transducer, as defined in claim 9, wherein,

said vibratile diaphragm is prepared at normal room temperature. 11. The method of fabricating an electret transducer as defined in claim 9, wherein,

said dielectric plate employed in preparing said vibratile diaphragm comprises at least two sheets of soda-lime glass, each approximately 0.1 cm

thick and each exhibiting a resistance of at least 10 ohms.

Y UNITEh STATES PATENTQFMQE t CER'HWCATE F QfiRRECTl-"N Patent No. 3,705 ,312 v *Dat December 5 1972 Inventor-( s) Gerhard; Martin Sessler and James Edward West It is oertified. that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 4, line &0, under the second columnof the table, headed "Effective Surface-Charge Densitjyflxthe second numeral listed as "-1.0" should read -l.2'-.

' Column 5, line 17-, the word "sandwich" should read -sandwiohed--n-.

Signed and sealed this 29th day of May 1973-.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Q v ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents qr FORM P071050 USCOMM-DC 60376-P69 .5. GOVERNMENT PRINTING OFFICE 2 969 O-3GG3S" 1 -J UNITED STATES rATENTeFmcE-P f' v QERTIWQAZEE @E QQRREQW-QN PatentlNol- 3,7 5,31 I *Dated December 5. 1972.: Inventor-(s) Gerhard Martin Sessler and James Edward West It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column l, line 10, under the second column of the table,

' headed "Effective Surface-Charge Densityflrthe second numeral listed as "-1.0" should read l.2

Column 5, line 17-, the word "sandwich" should read --sandwiche d-.

- Signed and sealed this 29th day of May 1973-.

(SEAL) Attest:

EDWARD M.FLETCHER,JRQ a ROBERT GOTTSCHALK Attesti'ng Officer Commissioner of Patents FORM PO-1050 (10 69) a USCOMM-DC 60376-P69 1 i U.S. GOVERNMENT PRINTING OFFICE l95 0-366-334, 

2. Apparatus for electrically charging a thin polymer foil, as defined in claim 1, wherein, said auxiliary dielectric insert comprises at least one sheet of soda-lime glass approximately 0.1 cm thick.
 3. Apparatus for electrostatically charging a thin polymer foil, as defined in claim 1, wherein, the thickness of said polymer foil and the thickness of said auxiliary dielectric insert are proportioned to establish a steady state current density within the critical range of about 10 8 to 10 6 A/cm2.
 4. Apparatus for electrostatically charging a thin polymer foil, as defined in claim 1, wherein, said auxiliary dielectric insert has a resistance in the range 109 to 1012 ohms for an area of 10 to 100 cm2.
 5. Apparatus for electrostatically charging a thin polymer foil, as defined in claim 1, wherein, said auxiliary dielectric insert has a resistance sufficiently high to induce breakdown of said foil but low enough to allow a relatively high voltage to be developed across said polymer foil.
 6. Apparatus for electrostatically charging a thin polymer foil, as defined in claim 1, wherein, said voltage applied between said electrodes is of approximately 30 kV.
 7. The method of fabricating a foil electret characterized by a charge density in excess of 0.5 X 10 6 C/cm2, which comprises the steps of: sandwiching a foil of polymer material and an auxiliary dielectric plate having a resistance sufficiently high to permit nondestructive breakdown of said foil, together between a pair of plane conductive electrodes, compressing said electrodes to hold said sandwich of foil and dielectric in close contact, applying a sufficiently high voltage between said electrodes to cause said foil to break down in localized channels, but not sufficiently high to cause said dielectric plate to break down, and maintaining said voltage between said electrodes for an interval of about one minute to charge areas of said foil remote from said channels.
 8. A foil electret characterized by a charge density in excess of 0.5 X 10 6 C/cm2 prepared in accordance with the method defined in claim
 7. 9. The method of fabricating an electret transducer, which comprises the steps of: preparing an electret diaphragm from a thin polymer film metallized on one of its surfaces by supporting said film between two electrodes, inserting a dielectric plate having a resistance sufficiently high to permit nondestructive breakdown of said film between the nonmetallized surface of said film and one of said electrodes, and supplying a voltage of approximately 30 keV between said electrodes for about 1 minute; and supporting said electret diaphragm in juxtaposition to a conductive backplate.
 10. The method of fabricating an electret transducer, as defined in claim 9, wherein, said vibratile diaphragm is prepared at normal room temperature.
 11. The method of fabricating an electret transducer as defined in claim 9, wherein, said dielectric plate employed in preparing said vibratile diaphragm comprises at least two sheets of soda-lime glass, each approximately 0.1 cm thick and each exhibiting a resistance of at least 1010 ohms. 